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Review
Underground coal gasication: From fundamentals to applications
Abdul Waheed Bhutto a, Aqeel Ahmed Bazmi b,c, Gholamreza Zahedi b,*
a Department of Chemical Engineering, Dawood College of Engineering & Technology, Karachi, Pakistanb Process Systems Engineering Centre (PROSPECT), Chemical Engineering Department, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, Skudai 81310,
Johor Bahru (JB), Malaysiac Biomass Conversion Research Centre (BCRC), Department of Chemical Engineering, COMSATS Institute of Information Technology, Lahore, Pakistan
a r t i c l e i n f o
Article history:
Received 4 April 2012
Accepted 10 September 2012
Available online 22 October 2012
Keywords:
Underground coal gasication
UCG kinetics
Gasier operation
Post-burn coal processing
Coal drilling
a b s t r a c t
Underground coal gasication (UCG) is a promising option for the future use of un-worked coal. UCG
permits coal to be gasied in situ within the coal seam, via a matrix of wells. The coal is ignited and air is
injected underground to sustain a re, which is essentially used to mine the coal and produce
a combustible synthetic gas which can be used for industrial heating, power generation or the manu-
facture of hydrogen, synthetic natural gas or diesel fuel. As compared with conventional mining and
surface gasication, UCG promises lower capital/operating costs and also has other advantages, such as
no human labor underground. In addition, UCG has the potential to be linked with carbon capture and
sequestration. The increasing demand for energy, depletion of oil, and gas resources, and threat of global
climate change have lead to growing interest in UCG throughout the world. The potential for UCG to
access low grade, inaccessible coal resources and convert them commercially and competitively into
syngas is enormous, with potential applications in power, fuel, and chemical production. This article
reviews the literature on UCG and research contributions are reported UCG with main emphasis given to
the chemical and physical characteristic of feedstock, process chemistry, gasier designs, and operating
conditions. This is done to provide a general background and allow the reader to understand the
inuence of operating variables on UCG. Thermodynamic studies of UCG with emphasis on gasier
operation optimization based on thermodynamics, biomass gasication reaction engineering andparticularly recently developed kinetic models, advantages and the technical challenges for UCG, and
nally, the future prospects for UCG technology are also reviewed.
2012 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .190
2. Underground coal gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .193
2.1. UCG for synthetic fuel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
2.2. Process overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
2.2.1. Chemical processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
2.2.2. Physical process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
2.2.3. Effect of coal reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
2.2.4. Gasifying agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
2.2.5. Effect of pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
2.2.6. Effect of heat loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
2.2.7. Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
2.2.8. Cavity growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
2.2.9. Gas diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
2.2.10. Velocity of combustion front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
2.2.11. Compositions of syngas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
2.2.12. Optimization of UGC operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
* Corresponding author. Tel.: 60 7 553583; fax: 60 7 5566177.
E-mail addresses:grzahedi@cheme.utm.my,grzahedi@yahoo.com(G. Zahedi).
Contents lists available atSciVerse ScienceDirect
Progress in Energy and Combustion Science
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0360-1285/$e see front matter 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.pecs.2012.09.004
Progress in Energy and Combustion Science 39 (2013) 189e214
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3. Thermodynamics of UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .201
3.1. Thermodynamic equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
3.2. Carbon-oxygen steam equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
3.3. Cold gas efficiency (hcg) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
4. Kinetic studies of UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
4.1. Single First Order Reaction model (SFORM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
4.2. Distributed activation energy model (DAEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
4.3. Reactions of formation of selected gas products in coal pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
4.4. Order of reaction and activation energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2074.5. Rate controlling step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
4.6. Chemical reaction rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
5. Challenges for UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
5.1. Suitable site selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
5.2. Technical challenges for UCG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
5.2.1. The major issues in the use of UCG technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
5.2.2. Exploration of the UCG site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
5.2.3. Choice of a suitable drilling technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
5.2.4. Environment and safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
6. Advantages of underground coal gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
6.1. Advantages of underground coal gasification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
6.2. UGC challenge and promises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
6.3. UCG-CCS concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
6.4. Post-burn processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
6.5. Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2107. Conclusions and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
1. Introduction
Coal, a fossil fuel created from the remains of plants that lived
and died about 100e400 million years ago when parts of the Earth
were covered with huge swampy forests is classied as a nonre-
newable energy source because it takes millions of years to form.
Coal has been used as a source of energy for nearly 3000 years.
Although mined in Europe as early as the 13th century, it was not
a highly desirable fuel because of its toxic combustion products.Coal did notbecome an important source of fuel until the beginning
of the Industrial Revolution about 300 years later.
The coal industrys largest environmental challenge is removing
organic sulfur, a substance that is chemically bound to coal.
Traditional methods of burning coal produce emissions that can
reduce air and water quality. Clean coal technologies remove sulfur
and nitrogen oxides before, during, and after coal is burned, or
convert coal to a gas or liquid fuel. Fluidized Bed Combustion is
a clean coal technology which keeps both sulfur and nitrogen
oxides in check. Coal Gasication is another clean coal technology
bypasses the conventional coal burning process altogether by
converting coal into a gas. This method removes sulfur, nitrogen
compounds and particulates, before the fuel is burned, making it as
clean as natural gas. Coal was rst used in gas production duringthe late 18th century. Early production was used primarily for
lighting, but as gasication techniques improved, applications grew
wider. By the 19th century the conversion of coal to gas was a well-
established commercial process.
Globally, coal will still remain an indispensable source of
chemical feedstock and energy for a long period of time. New and
improvedprocesses for itsefcient and environmentally acceptable
use will be a steady challenge for coming generations of coal
scientists and for society to support the research required [1].
World energy policy is gripped by a fallacy d the idea that coal
is destined to stay cheap for decades to come. This assumption
supports investment in clean-coaltechnology and trumps serious
efforts to increase energy conservation and develop alternative
energy sources. Underground coal gasi
cation (UCG) is a promising
option for the future use of un-worked coal. UCGd may eventually
make marginal coal reserves accessible, but it will take time and
substantial investment to be commercialized on a large-scale[2].
Most current technologies of coal gasication such as entrainedow,uidized bed, and moving bed use a surface reactor for gasi-
cation. The main differences between these technologies relate to
the gas ow conguration, coal particle size, ash handling, and
process conditions [3]. An alternative for surface gasier is an
underground coal gasier. UCG is a is a combination of mining,exploitation and gasication that eliminates the need for mining
and can be used in deep or steeply dipping, unmineable coal seam
.UCG is an in situ technique to recover the fuel or feedstock value of
coal that is not economically available through conventional
recovery technologies. It has been regarded to be an important way
to utilize low-rank and unmineable coals. The international expe-
riences in the modeling and the experimental tests of underground
coal gasication (UCG) show that UCG process offers an attractive
option of utilizing unmineable coal[4e21]. Probably the strongest
appeal of underground coal gasication at present is its potential
value in exploiting marginal coal reserves that otherwise would
remain unrecoverable[22].
Coal reserves signicantly exceed those of oil and gas. Worlds
coal distribution on land is shown in Fig. 1. When coal resourcetotals is considered (including coal which it is uneconomic to
mine), it dominates the fossil fuel picture. Estimates of total world
coal resource (including unmineable coal) are usually stated in
trillions of tons rather than billions. Recent estimates of the total
remaining coal resource in the world quote a gure of 18 trillion
tons[23].
Today, less than one sixth of the world s coal is economically
accessible. The chances of countries around the world choosing not
to use this coal resource are very low indeed but unless cleaner and
cheaper ways can be found to convert coal to gas or liquid fuels,coal
is unlikely to become an acceptable replacement for dwindling and
uncertain supplies of oil and natural gas. Underground coal gasi-
cation (UCG), taken on its own, offers the prospect of increasing the
world
s usable coal reserves by a factorof at least three.Fortunately,
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potential sites for UCG operations correspond to locations where
sites are plentiful for sequestering CO2 in geologic formations
underground. UCG also enhances the storage capacity of the coal
seam itself to store injected CO2. The generated gas, called syngas,
would be taken from the ground and the by-products separated
out. The CO2would then be returned downhole nearby.
The purpose of underground gasication of coal, regardless of
method used, is to obtain the energy contained in the fuel for use
on the surface, without mining in the usual sense of the term.
Underground gasication can be described as (1) a process where
coal, in place, is consumed by partial combustion with air, oxygen,
steam, or any combination of these to produce a low caloric value
gas (80e300 Btu per cu ft) or (2) a complete combustion process in
which air is used to produce a gas containing carbon dioxide,
nitrogen, and considerable thermal energy[25].
UCG also lowers the capital investment by eliminating the need
for specialized coal processing (transporting and stocking) and
gasication reactors. UCG has other advantages such as increased
Fig. 2. Current world-wide status of UCG technology: Map shows underground coal gasication (UCG) sites worldwide, including planned sites and prior pilot test sites, current
international UCG activities overlaying CO2 storage potential areas. Gray areas show potential areas for geological carbon storage [23,31,32].
Fig. 1. Worlds coal distribution black areas on land: Map excludes Antarctica, which contains large coal deposits but is not usable by international convention [24].
A.W. Bhutto e t al. / Progress in Energy and Combustion Science 39 (2013) 189e214 191
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work safety, no surface disposal of ash, low dust, and noise pollu-
tion. It can be operated at high pressure to increase the reaction
intensity and improve the efciency of the process. UCG is partic-
ularly advantageous for deep coal deposits and steeply dipping coal
seams since at these conditions less gas leakages to the surround-
ings and high pressures favor methane formation The successful
application of such a process would provide a low to medium BTU
gas (100e
300BTU/SCF), depending on whether air or an oxygenesteam mixture is used[26]. Composition and heating value of the
product gas depends on the thermodynamic conditions of the
operation as well as on the composition and temperature of the
gasifying agent employed.
In order to avoid potential environmental concerns, the reactor
cavity is operated at less than hydrostatic pressure, which brings
water into the gasication reactor in situ. As such, successful UCG
operation relies on the natural permeability of the coal seam to
transmit gases to and from the combustion zone, or on enhanced
permeability created through reversed combustion, an in-seam
channel, or hydro-fracturing[27].
The rst recorded proposal for UCG was by Siemens. Sir William
Siemens, a German scientist, was credited with rst suggesting
underground coal gasi
cation in 1868 [28], followed by
Mendeleyev 20 years later. No further work was done until the
1930s, when an experimental station was started in the Donetsk
coaleld in the then Soviet Union, to be followed by commercial
installations in 1940 [29]. John et al. [25] has given excellent
bibliography of the literature on UCG between 1945 and 60.
Underground gasication continued at a number of locations in the
Soviet Union until the late 1970s, with production of some
25,000 million Nm3 of gas from around 6.6 million tons of coal.
Some of the well-documented UCG operations are those at Angren-
Uzbekistan, Queensland-Australia, Alberta-Canada, Walanchabi
City-China, Majuba-South Aferica.
A commercial-scale UCG plant is still being operated in Angren,
Uzbekistan, where gas of an average heating value of 3.1e3.5 kJ/m3
is produced in an air-blown gasication process. The UCG gas
produced is fed into a power station which is situated adjacent to
the Yerostigaz operation in Angren. Yerostigaz has produced this
gas to generate power at the 400 MW power station at Angren.
Operators drill wells to inject air or oxygen that drives combustion
and gasication in situ, and to produce the coal gas to surface for
further processing, transport, or utilization. The most advanced
UCG operation is at Chinchilla in Queensland, Australia, where the
operator claims to be generating electricity from UCG product gas
at a highly competitive cost (1.5 US cents per kWh).In October 2008, Carbon Energy successfully produced syngas
from its unique UCG module based on the parallel controlled
retractable injection point (CRIP) method. The trial, which ran for
100 days, reached coal gasication rates of around 150 tons per day
and produced a high-quality syngas. Since then, Carbon Energy has
installed two more modules and constructed a 5-MW electric
power plant to be fed with syngas from Module 2. Module 1 is
being carefully decommissioned. Plans for scaling up to 25 MW of
electricity generation are under way, and a second project in
Queensland, known as the Blue Gum Energy Park, is also in the
early stages of planning.
Swan Hills Synfuels recently produced syngas from its pilot
project in Alberta, Canada. This project is the deepest UCG pilot
ever undertaken, at a depth of 1400 m, and is using the linearcontrolled retractable injection point method. The ENN Group Co.
Ltd. (a subsidiary of the Xinao company) produced syngas from
a pilot project in Walanchabi City, Inner Mongolia, China, for 26
months, gasifying more than 100,000 tons of coal. Although not
much information has been made available about this project, it is
known that there were initially seven injection and production
wells, which were rstred in October 2007 using air. ENN is now
in its fourth year of operation at the plant.
The Majuba UCG project has been producing syngas since
January 2007 and began delivering UCG syngas to core with coal
at the Majuba Power Station in late 2010. The project contributes
about 3 MW to the overall output of 650 MW from the electric
power station using the linked vertical well method. This project is
now the longest running UCG trial in the western world. Plans arein place to expand the facilities to 1200 MWe output, with 30% of
the plants fuel provided by syngas.
There have been over 50 UCG tests or pilot operations world-
wide. Trials were carried out at depths in excess of 500 m by
a European consortium (UK, Spain and Belgium) between 1992 and
1998 at Teruel in Spain.Table 1summarizes the history of the UCG
and Fig. 2 illustrates the current world-wide status of the
technology.
The potential for UCG to access low grade, inaccessible coal
resources and convert them commercially and, competitively into
syngas is enormous, with potential applications in power, fuel, and
chemical production. UCG research and development have been
conducted in several countries, including long-term commercial
operation of several UCG plants in the former Soviet Union.
Fig. 3. Potential development of UCG: Step 1: well drilling and link establishment. Step
2: coal seam ignition and commencement of gasication and step 3: site clean-up by
ushing cavity with steam and water to remove potential contaminants [19].
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Information on UCG technology, however, is limited and there is
a lack of compact review articles in this area [33]. Most of the
current available literature on UCG emphasizes on its geological
implications, environmental concerns and numerical analysis,
modeling and simulation based on laboratory or pilot scale studies.However, and in spite of the signicance of all this, there is no
comprehensive review on UCG process description with emphasis
on its thermodynamic and kinetic studies. We believe that, in all
these respects, this is a timely contribution. We anticipate that this
review will promote research and development efforts, scale-up
of the gasication process, and large-scale implementation of
UCG in future.
In this article, research contributions are reported according to
the following sections:
In Section2, we reviewed the UCG with main emphasis given
to the chemical and physical characteristic of feedstock,
process chemistry, gasier designs, and operating conditions.
This is done to provide a general background and allow thereader to understand the inuence of operating variables on
UCG.
In Section3, we discussed thermodynamic studies of UCG with
emphasis on gasier operation optimization based on
thermodynamics.
In Section 4, we reviewed coal gasication reaction engi-
neering and particularly we reviewed the recently developed
kinetic models.
Section5 has discussed challenges for UCG and the proposed
approach which has been implemented to overcome the
existing challenges.
Section6 summarized the advantages and limitations of UCG
and in Section 7, we provide concluding remarks and future
prospects for UCG technology.
2. Underground coal gasication
2.1. UCG for synthetic fuel production
UCG permits coal to be gasied in situ within the coal seam, viaa matrixof wells. The coal is ignited and air is injected underground
to sustain a re, which is essentially used to produce and transport
combustible synthetic gas to surface. This synthetic gas can be used
for industrial heating, power generation or the manufacture of
hydrogen, synthetic natural gas or other fuels. As compared with
conventional mining and surface gasication, UCG promises lower
capital/operating costs and also has other advantages, such as no
human labor underground for coal mining. In addition, UCG has the
potential to be linked with carbon capture and sequestration [34].
The increasing demand for energy, depletion of oil and gas
resources, and threat of global climate change have led to growing
interest in UCG throughout the world.
The primary components of UCG syngas are H2, CO, CO2, CH4,
and H2S. The pressures and temperatures of produced gas aresimilar, at 30e50 bars for a 300e500 m deep seam, and 500e
800 C outlet temperatures for sub-bituminous coals and up to
1000 C for bituminous coals. The product gas requires cleaning
once it has reached the surface, either to meet the specication for
input into a gas turbine (for electricity generation), or to be of
sufcient purity for use as a chemical feedstock for conversion to
synthetic fuels.
2.2. Process overview
UCG has been approached in many different ways. The old
technique to gasify the coal in situ uses two-vertically drilled wells
as the Injection and Production wells. The procedure consists of
three steps as shown in Fig. 3. In the
rst step an injection and
Table 1
History of the UCG[30].
Test site Country Year Coal type Seam thickness (m) Seam depth (m) Dipa (degrees) Coal gasied (t) Syngas cv (mj/m3)
Lisichansk Russia 1934e36 Bit 0.75 24 N/A N/A 3e4
Lisichansk Ukraine 1943e63 Bit 0.4 400 0 N/A 3.2
Gorlovka Russia 1935e41 N/A 1.9 40 N/A N/A 6e10
Podmoskova Russia 1940e62 SBB 2 40 0 N/A 6 with O2Bois-la-Dame Belgium 1948 A 1 N/A N/A N/A N/A
Newman Spinney UK 1949e
59 SBB 1 75 N/A 180 2.6Yuzhno-Abinsk Russia 1955e89 Bit 2-Sep 138 60 2 mt 9e12.1
Angren Uzbekistan 1965enow SBB 4 110 N/A Over 10 mt 3.6
Hanna 1 USA 73e74 HVC 9.1 120 0 3130
Hanna 2 USA 75e76 HVC 9.1 84 0 7580 5.3
Hoe Creek 1 USA 1976 HVC 7.5 100 0 112 3.6
Hanna 3 USA 1977 HVC 9.1 84 0 2370 4.1
Hoe Creek 2A USA 1977 HVC 7.5 100 0 1820 3.4
Hoe Creek 2B USA 1977 HVC 7.5 100 0 60 9.0
Hanna 4 USA 77e79 HVC 9.1 100 0 4700 4.1
Hoe Creek 3A USA 1979 HVC 7.5 100 0 290 3.9
Hoe Creek 3B USA 1979 HVC 7.5 100 0 3190 6.9
Pricetown USA 1979 Bit 1.8 270 0 350 6.1
Rawlins 1A USA 1979 SBB 18 105 63 1330 5.6
Rawlins 1B USA 1979 SBB 18 105 63 169 8.1
Rawlins 2 USA 1979 SBB 18 130e180 63 7760 11.8
Brauy-en-Artois France 1981 A 1200 N/A
Thulin Belgium 1982e84 SA 860 N/A
Centralia Tono A USA 84e85 SBB 6 75 14 190 9.7
Centralia Tono B USA 84e85 SBB 6 75 14 390 8.4
Haute-Duele France 1985e86 A 2 880
Thulin Belgium 1986e87 SA 6 860 157
Rocky Mountain 1A USA 87e88 SBB 7 110 0 11200 9.5
Rocky Mountain 1B USA 87e88 SBB 7 110 0 4440 8.8
El Tremedal Spain 1997 SBB 2 600
HVC High Vol Bit, Bit Bituminous, SBB Sub Bituminous, SA Semi-anthracite, A Anthracite.a Dip is the maximum angle between the inclined plane and the horizontal plane. Dip is always perpendicular to strike, and has both a compass direction and an angle.
Inclinometer is used to measure the amount of dip in degrees (a plane lying at along the horizontal as zero dip).
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production well are drilled from the surface to the coal seam and
highly permeable path within the coal seams are established
between these two well.Prior to the gasication step a linkage path
is created between injector and producer. Several techniques can be
used for linking the wells, including the Reverse Combustion
Linking (RCL), Forward Combustion Linking (FCL), hydro-fracking,
electro-linking, explosive and in-seam linking. Other techniques
for the in situ gasication include CRIPs, long and large tunnel
gasication, and two-stage UCG[35e37].
The RCL is a method of linking which includes injection of an
oxidant into one well and ignition of coal in the other so that
combustion propagates toward the source of oxidant as shown in
Fig. 4(a).
In the course of the FCL coal is ignited in the injection well, and
the re propagates toward the production well as shown in
Fig. 4(b). During forward gasication, the ame working face
gradually moves to the outlet, making the dry distillation zone
shorter and shorter. At the time when forward gasication is nearly
complete, the reduction zone also becomes shorter[38].
Flow of oxidant into the injection well is maintained until the
rereaches the bottomof the injection wellin the RCL or that of the
production well in the FCL. This outcome is accompanied by
a signicant drop in the injection pressure indicating creation ofa low hydraulic resistance link between the wells, which estab-
lishes a low hydraulic resistance path between the two wells.
CRIP technique is suitable for thin, deep coal seams, replaces the
vertical injector by a horizontal injector [39]. During the gasica-
tion process, the burning zone grows in the upstream direction, in
contrast to the gas ow in the horizontal direction. This occurs by
cutting off or perforating the injection linear at successive new
upstream locations. The CRIP technique produces higher quality
gas, results in lower heat loss than the two-vertical well congu-
ration, and improves the overall efciency of the UCG process[40].
Once a successful link has been established the second step is
ignited. The gasication step starts with ignition of the coal and the
injection of air or air enriched with oxygen. Both permeable bed
gasication and natural convection driven surface gasication will
occur. When the gas quality deteriorates the injection well is burnt
to allow injection further upstream.
Gasication occurs when a mixture of air or oxygen and steam is
forced into the coal seam through injection well and react chemi-
cally with the coal, generating a synthesis gas, which is recovered
through product well. At the surface the raw product gas is cleaned
for industrial uses[20].
As gasication proceeds, an underground cavity is formed.
Water from the surrounding strata will enter the cavity and
participate in the gasication process leading to a drop in the local
water table. At some point, the coal in the vicinity of the injection
well will be exhausted and steps one and two will be repeated to
access fresh coal to sustain gas production. In the commercial
operations several underground gasiers will be operated simul-
taneously. Once the gasication operations in a section of coal seam
have nished, the third step is to return environment back to its
original state. This is achieved by ushing the cavities with steam
and/or water to remove pollutants from cal seams to prevent them
from diffusing into surrounding water aquifers. Over the time, the
water table will return to a level close to that existing prior to the
start of gasication[20]. The composition of the product gas from
UCG can very substantially depending on the injected oxidant used,
operating pressure and mass and energy balance of the under-ground reactor.
CRIP technique,is suitable forthin, deep coal seams,replaces the
vertical injector by a horizontal injector [39]. The CRIP method
requires two horizontal wells drilled along a coal seam. One is near
the top of the seam and the other near the bottom. The bottom
(injection) well is lined with metal pipe. The upper well is the
production well. As pyrolysis proceeds, the burn cavity moves
toward the base of the wells, progressively exposing more and
more of the injection pipe. At an appropriate time, the pipe is
melted or burned off and a new period of pyrolysis begins. In effect,
the old problems of well plugging are circumvented by simply
starting a newburn periodically along the horizontal wells [41]. The
CRIP method was rst tried successfully in early 1982 with a three-
day trial, gasifying a 40-ton cavity. The injection pipe was thenburned off and a second 10-ton cavity started. The original cavity
cooled to 500 C, and the second achieved the typical operating
temperature of 1000 C. The average heating values of the product
gases were between 265 and 277 Btu per standard cubic foot.
Burning is started by pyrophoric silane and propane gases. The
silane ignites upon encountering the oxygen in the burn cavity and
burns long enough to subsequently ignite the propane, which is
injected into the well. The propane actually ignites the coal in the
cavity. At a suitable time, the propane is shut off and the pyrolysis
sustains itself. This method has proved reliable since its adoption.
Burning can also be started by passing LPG through the injection
well for a short period of time (3e5 min) to initiate the combustion.
An electric spark is generated for ignition of the liqueed petro-
leum gas (LPG) in the channel of the coal block near the mouth ofthe injection well. Once coal is ignited, the LPG supply is stopped
and oxygen is continuously passed through the channel created in
the coal block until the completion of the experiment[42].
CRIP technique uses a combination of conventional and direc-
tional drilling to drill the process wells. First, the vertically-drilled
Production Well is drilled until it intersects the coal seam. Then
the vertical section of the Injection Well is drilled to a pre-
determined depth, after which directional drilling is used to
deviate the hole and drill along the coal seam until it intersects the
Production Well. This technique enables the injection point (i.e. the
end of the coiled tubing) to be retracted back along the coal seam,
which is of benet because it allows for fresh coal to be accessed
each time the syngas quality drops as a result of cavity maturation.
Retraction of the injection point along the coal seam is known as
Fig. 4. Schematic views of the reverse and forward combustion linking in UCG. (a)
Reverse combustion linking. (b) Forward combustion linking [36].
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a CRIP maneuver, and between 10 and 20 such maneuvers are ex-
pected during the course of a modules lifetime. Directional drilling
is a proven technology in the oil and gas industry.
The in-seam drilling of coal seams has been part of coal
exploitation since at least the 1950s. Underground steering of
boreholes made its commercial entrance in the oil and gas industry
around 1990, when operators established the benets of lateral
drilling for extending the life of wells and xed drilling platforms
and for reaching inaccessible locations. Nowadays directional
drilling has become common for coal bed methane (CBM) and
enhanced CBM applications; there are specialist drilling companies
around who supply services to CBM operators. The focus to-date
has been on reducing costs. UCG has a tighter requirement on
accuracy. The ability of directional drilling to meet these require-
ments at an affordable cost is still under review [37]. The CRIP
technique produces higher quality gas, results in lower heat loss
than the two-vertical well conguration, and improves the overall
efciency of the UCG process[40].
Two-stage UCG is a technique of supplying air and steam
cyclically[10,43]. In the rst stage, air is supplied to make the coal
burn and store heat to produce air gas; in the second stage, steam is
suppliedto produce water gas. Only if sufcient heat isstored in the
rst stage can the decomposition reactions in the second stage runsmoothly and the water gas with high heating value be ensured.
Meanwhile, the degree of the coal layer decomposition and the
production volume of the gas are totally determined by the
temperature distribution in the coal layers[44].
During in situ coal gasication remote sensing technique
may be used for mapping underground fracture systems,
locating tunnels or water-bearing strata and mapping burn
fronts[45].
2.2.1. Chemical processes
The study considers the quasi-steady burning of a carbon
particle which undergoes gasication at its surface by chemical
reactions, followed by a homogeneous reaction in the gas phase.
The main chemical processes occurring during coal gasication aredrying, pyrolysis, combustion and gasication of the solid hydro-
carbon. These processes occur in all methods of coal gasication,
whether conducted in surface gasiers or in situ. Fromthe chemical
and thermodynamic point of view, the UCG process runs analogi-
cally to gasication in the surface reactors [46]. The most important
chemical reactions taking place during underground coal gasica-
tion are listed inTable 2.
Chemical reactions (1)e(4) take place on the wall plane of the
coal seams (heterogeneous reactions), while (6) and (7) reactions
occur at the gaseous stage (homogeneous reactions).
In addition to these listed, reactions involving nitrogen and
sulfur are also important. The nal product gas consists of
hydrogen, carbon monoxide, carbon dioxide, methane and
nitrogen. Composition and heating value of the product gasdepends on the thermodynamic conditions of the operation as well
as on the composition and temperature of the gasifying agent
employed[46].
Duringin situ combustion of coal different processes of vapor-
ization (drying), pyrolysis, and combustion and gasication of char
take place collectively. The UCG process has a zonal character and
the main gasication reactions occur both in the solid and the
gaseous phases as well as on their boundaries. Qualitative
description of phenomena at the UCG cavity wall is explained
inFig. 5.
In the solid phase mainly the pyrolysis and the drying processes
take place. Along with the migration of the gaseous product of the
thermal decomposition through the pores and slots of the solid
phase, various homo- and heterogenic reactions occur. The rates of
these processes depend mostly on the temperature. On the phase
boundary in the gasication channel heterogenic reactions take
place. Their rates aredetermined by the diffusion and the accessible
reaction area. The major products of the reaction of oxygen with
carbon in the gasication area (oxidation zone) are carbon dioxide
and carbon monoxide[46].
Based on the differences in major chemical reactions, the
temperature, and the gas compositions, the gasication channelcan be divided into three zones: oxidization zone, reduction zone
and dry distillation zone as shown in Fig. 6[21]. In the oxidization
zone, the multi-phase chemical reactions between oxygen con-
tained in the gasication agent and the carbon in the coal seam
occur, producing heat and making the coal seam very-hot. The
coal seams become incandescent with temperature ranging
from 900 C to 1450 C [47]. Inherent water plays a role in coal
oxidation, affecting oxygen transport within coal pores and
participating in the chemical reactions during the oxidation
process. Details of chemical reactions involving water have not yet
been elucidated[48].
With the O2burning up gradually, the air stream gets into the
reduction zone. In the reduction zone H2O(g) and CO2 are
reduced to H2 and CO under the effect of high temperature,when they meet with the incandescent coal seams. The
temperature ranges from 600 C to 1000 C, and the length is
1.5e2 times that of the oxidation zone with its pressure being
0.01e0.2 MPa [49]. Additionally, under the catalytic action of
coal ash and metallic oxides, a certain methanation reaction
occurs [Eq. (4)]. The above endothermic reactions cause the
temperature at the reduction zone to drop until it is low enough
to terminate the reduction reactions.
After the endothermic reactions in the reduction zone, the
gas current temperature drops, and then it begins to ow into
the destructive distillation and dry zone (200Ce600 C).
The main physical changes for coal with high water content
are dewatering and cracking, as well as absorption and
contraction of the coal, when the temperature is below 100
C.When the temperature is not higher than 300 C, only small
amounts of parafn hydrocarbon, water, and CO2 are separated
out. Over 300 C, the slow chemical changes take place,
accompanied with a light polymerization and depolymerization.
In the meantime, appropriate amounts of volatile and oil-like
liquid are separated out, which take on a gelatinous state
afterward. When the temperature of the coal seam rises to
350Ce550 C, a large proportion of tar oil is separated out
(500 C at its peak) and a certain amount of combustible gas is
yielded. The hydrocarbon gas is given out when the temperature
stands at 450Ce500 C. As the temperature of the coal seam
continues to rise until it is over 550 C, semi-coke remains begin
to solidify and contract, accompanied with the yield of H2, CO2,
and CH4 [47,50].
Table 2
Chemical reactions taking place during underground coal gasication.
Reaction equation React ion
rate (Ri)
DHo298
(MJ/kmol)
Equation
number
C O2/CO2 R1 393.8 (1)
C CO2/2CO2 R2 162.4 (2)
C H2O/H2 CO R3 131.4 (3)
C 2H2/CH4 R4 74.9 (4)
CO 1
2O2/CO2
R5 285.1 (5)
H2 1
2O2/H2O
R6 0.242 (6)
CO H2O/CO H2 R7 0.041 (7)
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The overall UCG process is strongly exothermic, and tempera-
tures in the burn zone are likely to occasionally exceed 900 C. Even
after cooling (through conductive heat loss to surrounding strata
and convective heat loss to native groundwater), syngas typically
ows through production wells at temperatures between 200 C
and 400 C. Around the burn zone, the high buoyancy of hot syngas
relative to groundwater will tend to lead to large pores getting
invaded with bubbles of syngas, which will heat the groundwater
and turn it into steam. A dynamic interface between steam and hot
groundwater will develop around the UCG burn zone, in which
steam will mix with the syngas[23].
Passing through these three reaction zones, the gas with the
main combustible compositions of CO, H2 and CH4 is formed, whose
proportion of contents varies from one gasication agent and air
injection method to another. These three zones move toward theoutlet along the direction of the airow, which, in turn, ensures the
continuous run of the gasication reactions[21].
Figs. 6 and 7illustrate different chemical regions of gasication
of coal in situ. In the drying zone, surface water in the wet coal is
vaporized at temperatures above the saturation temperature of
seam water at a specied pressure, which makes the coal more
porous. The dried coal undergoes the pyrolysis process upon more
heating in the next phase. During pyrolysis, coal loses about 40e
50% of its dry weight as low molecular weight gases, chemical
water, light hydrocarbons and heavy tars, and after evolving the
volatile matters, a more permeable solid substance called char will
be combusted and gasied by the injected oxidant agents and
exhausted gases from the previous steps [51,52]. The rates of the
gaseous phase reactions are determined mostly by the temperature
and concentration of the particular gaseous compounds. Develop-
ment of these reactions is frequently supported by the catalytic
inuence of some chemical compounds, e.g. iron oxides.
2.2.2. Physical process
In the process of underground coal gasication (UCG), the gas
movement not only inuences the concentration distribution and
movement ofuid in the burning zone directly, but also restricts
the diffusion of the gasication agent in the whole gasier.
Fig. 5. Qualitative description of phenomena at the UCG cavity wall [16,19].
Fig. 6. Division of gasication channel into three zones: oxidization zone, reduction
zone and dry distillation zone[21].
Fig. 7. Thermal wave propagation through coal seam during in situ gasication which
demonstrates the different regions[3].
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Therefore, it eventually determines the rate of chemical reaction
between gas and solid, and the process of burning and gasication.
Evidently, Lanhe 2003[15]suggested the study of moving patterns
ofuid in the gasier should precede the study of the process of
chemical reaction, the moving patterns of agents, and the distri-
bution regularity of temperature elds near the ame working
face.
In the process of underground coal gasication, under the effect
of high temperature, that a temperature eld forms in the coal layer
to be gasied within the coal and rock mass, which makes the
coal and rock layersdoriginally full of stratication, joints, and
fracturesdsoften, melt, cement, and solidify. Accordingly, the
internal molecular structure is rearranged and reorganized, which
leads to qualitative changes of organizational structure and
morphological appearance. Hence, obvious changes take place in
the physicomechanical properties of the coal and rock mass.
In the process of underground coal gasication, a high
temperature eld comes into being in the coal body under the high
temperature, which makes the coal seam, full of layers and joints
and interstices, soften, melt, glue, and solidify. Under the high
temperature, the internal molecular structure reorganizes, which
completely changes the coal seams surface morphology. Hence,
dramatic changes take place in the physical and mechanical prop-erties of the coal body. As a result, its corresponding physical and
mechanical properties are no longer constants, but functions of
temperature. The differences in the heat expansion coefcient
among coal grains and anisotropy generate new cracks, whose
extension leads to the connected net structure. Thus, all these
improve the connectivity of the pore passageway and increase the
seepage pressure of the dry distillation gas[53].
Research indicates that, under the non-isothermal condition,
the densities of the solid media and pore water are greatly affected
by the temperature and pressure [49]. However, the small defor-
mation of the solid skeleton still produces a certain effect on the
distribution of the temperature eld and seepage of underground
water in the gasication panel. Therefore, the deformation of
the solid particle is not negligible and can be regarded ascompressible [9].
The coal rock is extended and deformed by the pore uid
pressure. The uid inside the pores affects the cracks inside the
skeleton of the coal rock and the pores opening and closing;
second, the relation between the stressand strain of the coal rock is
changed by the uid in the pores, which in turn changes the elastic
modulus and compressive strength of the coal rock [54e56]. The
changes in the temperatureeld of the coal seam are due mainly to
the ame working face. When the temperature in the coal seam
rises, the desorption rate of the dry distillation gas in the coal seam
accelerates. The free dry distillation gas content in the coal
increases. The mass of the dry distillation gas which participates in
the seepage increases too. On the other hand, with the rise of the
temperature, the amount of absorbed dry distillation gas in the coalseam drops.
2.2.2.1. Operating conditions. The investigation by Perkins and
Sahajwalla[18]has found that the operating conditions that have
the greatest impact on cavity growth rate are temperature, water
inux, pressure, and gas composition in underground coal gasi-cation. In this section, the effect of operating conditions and coal
properties, namely, coal reactivity, operating pressure, heat loss,
and the type of oxidant used are investigated [16]. Lanhe [13]
while establishing the mathematical models on the under-
ground coal gasication in steep coal seams according to their
storage conditions and features of gas production process
concludes that numerical simulation on the temperature eld,
concentration
eld and pressure
eld is reasonable in the
underground gasication of steep coal seams on the experimental
condition.
2.2.2.2. The thickness of coal layers. UCG is inuenced by several
natural factors as described inTable 3. Most UCG operations were
carried out in more gas permeable conditions of brown coal beds
and younger formations of hard coals. Generally, these deposits
occurredat shallower depths, down to 300 m, and ignited relatively
easily. Strongly swelling and coking coals have the tendency to
block gas ow through the coal bed, thus hindering the course of
the reaction. The gasication of beds 1 m thick or more improves
economics [57]. Beds that are thinner than 0.5 m are not considered
suitable for UCG.
In the process of UCG, the burning area and gas are not only
cooled down through heat exchange but a part of the heat is also
lost into the coal seam and surrounding rocks (oor, roof), thus
having an adverse effect on the stability of the underground gasi-cation process. Eliot [58] suggested that when the thickness of
coal seam is smaller than 2 m, the cooling action with a dramatic
change for surrounding rocks affects the heat value of coal gas
considerably. As for comparatively thin coal seam, enhancing the
blowing velocity or oxygen-enriched blowing can improve the
heating value of gas. In the former Soviet Union, Lischansk under-ground gasication station adopted oxygen-enriched blowing in
the coal seam, for which the thickness is less than 2 m[58]. When
the thickness of coal layers is decreased or the intake rate of water
is increased, the CO2content in the gas will rise [58,59].
2.2.3. Effect of coal reactivity
The chemical reactivity of the coal is potentially very important
for UCG. The reported intrinsic reactivities of low rank coals differ
by up to 4 ordersof magnitudewhen extrapolated to typical gasier
operating temperatures[18]. The coal intrinsic reactivity has a big
impact on the distributions in the gasier and on the nal product
gas. In particular, high reactivity favors the production of methane
via the char-H2 reaction. Because this reaction is exothermic, the
increased reactivity for this reaction can lead to big changes in thenal product gas caloric value.
2.2.4. Gasifying agents
Gasication under different atmospheres such as air, steam,
steam-oxygen, and carbon dioxide has been reported in the liter-
ature. In general, the gasier atmosphere determines the caloric
Table 3
Classication criteria for UCG.
Criterion Characteristics/remarks
Coal type Any
Physicochemical properties of coal Recommended: high content of volatile
matter, low agglomerating capacityor its lack, ash content < 50% by weight
Occurrence depth Protability criterion
Bed thickness More than 1 m
Angle of inclination of coal bed Any
Type and tightness of rock mass Recommended: rmness and tightness
of rock mass, thickness and lithology
of rock massdoverburden in slightly
permeable layers (clays, silts, shale clays)
Hydrogeological conditions Recommended: lack ofssures, faults,
aquiferous layers, water reservoirs causing
water inow
D eposit tec tonic s Recommend ed homogeneity of deposi t
(lack ofssure, faults)
Quantity of resources Protability criterion
Methane presence in the bed Causes gas hazard
Conditions of infrastructure Recommended lack of building development
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value of the syngas produced. When one uses air as the gasifying
agent, a syngas with low heating value is obtained. This is mainly
due to the syngas dilution by the nitrogen contained in air.
However, if one uses steam or a combination of steam and oxygen,
a syngas with a medium caloric value is produced. Adding steam
changes carbon-oxygen system balance to carbon-oxygen-steam
system balance in the combustion process. Oxygen-steam gasi-
cation not only utilizes the surplus heat to improve the energy
efciency of the process, but also increases the gas production
volume per ton of coal and lowers the oxygen consumption volume
per ton of coal. The changing relationships between gas composi-
tions and steam/oxygen ratios are shown in Fig. 8[60].
The experiment results show that pure-oxygen underground
coal gasication, the water in the coal seams, or the leaching water
on the roof can be used to produce water gas. However, because
water evaporation consumes heat, and it is impossible to control
steam volume, gas compositions often present the wide uctua-
tions. Therefore, it is required to adjust the oxygen supplying
volume so as to keep the stable proceeding of gasication process.
FromFig. 8, it can be seen that with the rise in the steam/oxygen
ratio, the volume of steam increases, the H2content in the coal gas
improves, the CO content drops, and the CH4 content is heightening
a little[60].The syngas produced has a by UCG process has low caloric
value approximately one-eighth of natural gas if air injection is
used, and double this gure if oxygen injection is used. Oxygen-
enriched steam forward gasication has remarkable effects on gas
compositions. Under the testing environment, in pure oxygen
gasication, the average rising rate for the temperature of the
gasied coal seams is about 2.10 C/h; in the oxygen-enriched
steam forward gasication phase, the high temperature eld
mainly concentrates around gasication gallery, and the highest
temperature in oxidation zone reaches over 1200 C[61].
The air injected into a gasication channel is at a low speed, theame tends to propagate toward the injection point but, if the air
ow rate increases, the cavity tends to grow in the downstream
direction. It is also known that ame propagation is faster whenoxygen is used instead of air. This behavior is also expected since
oxygen-fed ames are hotter and have higher reaction rates[62].
Saulov et al. [62] considered the limit of high temperatures, high
activation energy and a strong air ow. Under these conditions the
surface of the channel has two zones, cold and hot. The tempera-
ture is insufciently high in the cold zone to initiate reactions,
while in the hot zone any oxygen on the surface reacts instantly.
Since the activation energy is high, these zones are separated only
by a very small distance. The overall reaction rate is determined by
the rate of diffusion of oxygen to the hot zone, while the oxygen
concentration on hotwalls is essentially zero. Under such condi-
tions the turbulent ame is fully controlled by diffusion and the
injection rate has no control over theame position. Combustion of
coal begins with devolitalization reactions at low temperatures and
can be cooled by the air stream. If these reactions play a noticeable
role in initiating the rest of the oxidation process or in the overall
energy balance, the ame position is affected by the air speed and
becomes controllable.
When other factors are the same, increases in ow rate and
operation time result inmonotonic increases in all the dimensions
of the cavity, and its volume. However, when the distance between
the injection and production wells is increased, the overall cavity
volume decreases, due to signicant reduction in the rate of growth
of the cavity in the forward direction[42].
2.2.5. Effect of pressure
Pressure is known to positively impact the performance of coal
gasication[63]. At close to atmospheric pressure, the gas caloric
value is very low because of the kinetic limitations of the gasi-
cation reactions. The changes in operating pressure can perfect the
underground gasication process to a great extent. Under thecyclically changing pressure condition, heat loss was obviously
reduced, and heat efciency and gasication efciency and the heat
value of the product gas are increased greatly. The underground
gasier with a long channel and big cross-section could improve
the combustion and gasication conditions to a large extent,
markedly bettering the quality of the product gas and the stability
of gas production. Therefore, the large-scale underground gasieris
a condition necessarily met by the industrial production[50].
2.2.6. Effect of heat loss
Heat losses from underground coal gasication are not easy to
estimate. If the cavity remains completely in the coal seam, then
heat losses to the surrounding strata will probably be small and can
be ignored. However, as the overburden is progressively exposed,irreversible heat loss to the surrounding will increase. It is not easy
to estimate this heat loss, because if the overburden undergoes
spalling, some of the energy used to heat it to cavity temperatures
may be recovered through preheating of the injected gas. The heat
loss mechanisms can probably be more easily investigated using
a dynamic model, in which cavity growth and heat loss are esti-
mated as functions of time, simultaneously.
2.2.7. Effect of temperature
The process of UCG is virtually one of a self-heat balance. The
heat produced by coal combustion contributes to the establishment
for ideal temperature eld in the underground gasier and also
leads to the occurrence of gasication reactions and, eventually, the
generation of gas.Temperature is a key factor in determining the continuous and
stable production in the process of underground coal gasication.
The patterns of variation for temperature eld in the gasier are
closely related to the nature of the gasication agent, gasication
modes, and the changes of cavity [8,49,61,64,65]. Under the pure
oxygen gasication condition, the average rising rate for the
temperature of the gasied coal seams is about 4.15 C/h; in the
oxygen-steam forward gasication phase the high temperature
eld mainly concentrates around loosening zones arising from the
thermal explosions, and the highest temperature in the oxidation
zone approaches 1300 C[6]. Compared with forward gasication,
the average temperature in the gasier for backward gasication is
lower [61]. The drop of temperature results in a decrease in CO
content while H2, CH4and CO2contents increases[50].Fig. 8. Gas composition variation with steam/oxygen (v/v)[60].
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In thermal-explosion gasication method, under the pure
oxygen gasication condition, the average rising rate for the
temperature of the gasied coal seams is about 4.15 C/h; in the
oxygen-steam forward gasication phase, temperatureeld mainly
concentrates around loosening zones arising from the thermal
explosions, and the highest temperature in the oxidation zone
approaches 1300 C. Test data showed that the forward oxygen-
steam gasication with moving points can obviously improve the
temperature conditions in the gasier. During the backward
oxygen-steam gasication, with the passage of time, the tempera-
ture of the gasication coal seams continuously increases,
approaches stable little by little, and was basically the same with
that of the forward gasication. Therefore, backward gasication
can form new temperature conditions and improve the gasication
efciency of the coal seams.
In the process of coal gasication, the changes of the tempera-
ture in the coal seam are due mainly to the heat transfer medium of
the ame working face, which corresponds to a source of heat[53].
In the process of underground coal gasication, the temperature of
coal seams around the gasication channel rises along with the
conducted heat. When the coal surface is heated by the hot gas or
the neighboring incandescent coal, its temperature distribution
expands toward the coal grains or the interior of the coal seam,which inevitably results in the thermal effects of absorption,
desorption, and seepage movement of dry distillation gas stored in
the coal seam [49,53,66,67]. King and Ertekin [68] study shows that
under non-isothermal conditions, either the absorption-desorption
process or the permeation-expansion process is linked to the
temperature.
According to the gasication theory, the temperature above
1000 C indicates a high-speed diffusion of the water decomposi-
tion reaction constituting the fundamental process for the
production of a hydrogen rich gas in the course of the UCG steam
stage. On the other hand, the temperature drop below 700 C
slowed down the reaction speed considerably. For these reasons,
special attention was paid to keeping parameters preferable for the
production of gas with a high content of the combustible compo-nents, mainly hydrogen. The oxygen stage was therefore continued
to achieve temperatures in the range between 1100 and 1200 C.
According to the simulated calculation results [13], with the
increase of the length for the gasication channel, the heating value
of the gas improves. However, behind the reduction zone, it
increases with a smaller margin. The inuence of the temperature
eld on the heating value for the gas is noticeable. Due to the effect
of temperature, in high temperature zone, the change of the
measured value of the concentrationeld for the gas compositions
is larger than that of calculated value.
The underground gasication of a large quantity of coal at
temperatures higher than 1000 C results in the typically argilla-
ceous overburden rocks overlying the coal becoming thermally
affected. Most of thermal reactions in argillaceous rocks are
endothermic.
2.2.8. Cavity growth
As the coal gasication reaction precedes a cavity consisting of
coal, char, ash, rubble, and void space, is created underground. The
size of the cavity formed during UCG impacts directly the economic
and environmental factors crucial to its success. Lateral dimensions
inuence resource recovery by determining the spacing between
modules, and ultimate overall dimensions dictate the hydrological
and subsidence response of the overburden. The exact shape and
size of the gasication channel during UGC are of vital importance
for the safety and stability of the upper parts of the geological
formation[69]. Due to upward growth the cavity eventually rea-
ches the interface between the coal seam and the overburden. From
that point onwards the development of the cavity can be strongly
inuenced by the interaction of the gas mixture with the over-
burden. At the start of the UCG process, typically, the exothermic
coal combustion reaction is required in order to create a sufciently
large underground cavity. In this early stage, cavity growth is
unconstrained by roof interactions. Once a stable temperature eld
is attained, steam is introduced in the cavity for gasication of the
coal in order to obtain the combustible product gases [38]. Theshape and rate of growth of this cavity will strongly impact other
important phenomena, such as reactant gas ow patterns, kinetics,
temperature proles, and so on [42]. The cavity size at any given
time depends on the rate of coal consumption and its shape
depends on the non-ideal ow patterns inside the cavity.
The cavity shape is almost symmetric around the injection well.
The cavity evolution behind the injectionwell (i.e.backward length)
is less than the height (inthe verticaldirection) and thewidthat the
injection point (in the transverse direction). The forward length of
the cavity (i.e. distance from injection well to the end point of the
cavity dome in the forward direction) is larger than the height and
the backward length. The convectiveuxof the reactantgases inthe
forward direction (i.e. toward the production well) contributes to
the additional growth of the cavity in this direction. The observednal cavity dome that is associated with a long outow channel is
nevertheless nearly hemispherical in shape.Fig. 9is a schematic of
thenal cavity shape, indicating the vertical, forward,backward and
transverse directions as dened here. The temperature at the cavity
roof is in the range of 950e1000 C whereas the oor temperature
varies between 650 and 700 C.
The volume of the cavity increases progressively with coal
consumption and thermomechanical spalling, if any, from the roof.
As the cavity growth is irregular in three dimensions, the ow
pattern inside the UCG cavity is highly non-ideal. The complexity
increases further because of several other processes occurring
simultaneously, such as heat transfer due to convection and radi-
ation, spalling, water intrusion from surrounding aquifers, several
Fig. 9. Schematic diagram de
ning forward length, backward length, height and width of the
nal cavity[42].
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chemical reactions, and other geological aspects [57]. Several
mathematical models have been developed considering the UCG
cavity as either a packed bed or a free channel Most of the existing
models consider the UCG cavity as a rectangular or cylindrical
channel[13,16,18,19,35,70e75].
Perkins and Sahajwalla [18] predicted cavity growth rate
between 1.6 and 5 cm/h using their mathematical model which
links linear cavity growth rates to reactivity and mass transport
properties. Daggupati et al. [38] measured the linear, vertical
growth rate of 1.1 cm/h (obtained using the measured cavity
heights at different times, with the other operating conditions
being the same).
The cavity volume is directly proportional to the coal
consumption whereas the shape depends on the relative rates of
growth taking place in each of the four identied representative
directions. While the coal consumption is governed by the extent or
rate of reaction that takes place in the cavity reactor, the growth in
each individual direction is a function of the complex reactant gas
ow eld inside the cavity, and other effects such as thermo
mechanical spalling of the coal. Chen et al. [69] has developed
model to calculate the temperature distribution in the vertical
direction, and the combustion volume.
According to the physical and chemical properties of coal andthe mining geology conditions of the burial for the coal seams, two
kinds of gasication channels can be formed in the gasication
panel; namely, free channel without solid phase and the percola-
tion patterned porous loose channel. In the longitudinal (or radial)
direction, the free channel can be divided into three zones (Fig.10),
i.e., free owing zone, reaction zone and the coal seams zone. The
gas phases ow under the condition of wall plane of the channel
continuously exchanging heat, consuming or producing certain
compositions. At the same time, the homogeneous reactions also
occur to the gas phases. In the reaction zone, the oxidation, reduced
reactions and the pyrolysis reactions of the coal occur. The heat
transfer to the gas phases, the consumption and production of the
compositions can be regarded as the boundary conditions for the
owing of the gas phases. In the coal seams zone, part of theheat inthe reaction zone loses in the coal seams mainly in the form of the
heat conduction, making the dry and distillation of the coal seams.
Therefore, we can observe the characteristics of the gas phase
moving and establish the control equation set of the free channel
gasication process.
The cavity growth directly impact on the coal resource recovery
and energy efciency and therefore the economic feasibility. Cavity
growth is also related to other potential design considerations
including avoiding surface subsidence and groundwater
contamination.
Installation of well pairs (injection and production wells) is
costly and therefore it is desirable to gasify the maximum volume of
coal between a well pair. As gasication proceeds, a cavity is formed
which will extend until the roof collapses. This roof collapse is
important as it aids the lateral growth of the gasier. Where the
roof is strong and fails to break, or where the broken ground is
blocky and poorly consolidated, some uid reactants will by-pass
the coal and the reactor efciency could decline rapidly. In
general, as depth increases, conditions should become increasingly
favorable to gasier development with a lower risk of bypass
problems occurring, except possibly in strong roof conditions[76].
2.2.9. Gas diffusion
Inthe process of combustion and gasication for the coal seams
in the gasier, the major reactions are multi-phase reactions. At
each stage of multi-phase reactions, the gas state reactant spreads
to the surface of the solid state reaction by the diffusion method.
Gas diffusion mainly has two kinds: molecular diffusion and
convection (eddy) diffusion. The process of the combustion for coal
seams depends on the gas diffusion features and the dynamic
characteristics for the chemical reactions. According to the
diffusion-dynamic theory for combustion [49], under the low
temperature condition, the overall velocity of the combustion andgasication process is mainly determined by the dynamics condi-
tions of the chemical reactions; under the high temperature
condition, the overall velocity of combustion and gasication
process mostly depends on the speed for oxygen to diffuse from the
main current to the carbon surface and the velocity of its product
diffusing from the carbon surface to the main current. Seeing from
the circumstances of the eld test of underground gasication and
model experiment, the temperature within the gasier (the vicinity
of the ame working face, in particular) is very high.
Moreover, considering the movement conditions for the uid,
we can conclude that the convection diffusion for gas is the
signicant factor inuencing the process of the underground
gasication. Under the condition of high temperature, molecular
diffusion results from the existence of concentration gradient,temperature gradient and pressure gradient[14].
While studying the basic features of convection diffusion for the
gas produced in underground coal gasication, on the basis of the
model experiment, through the analysis of the distribution and
patterns of variation for the uid concentrationeld in the process
of the combustion and gasication of the coal seams within the
gasier, Lanhe[14]established the 3-D non-linear unstable math-
ematical models on the convection diffusion for oxygen. Same
study concludes that oxygen concentration is in direct proportion
to its distance from the ame working face, i.e. the longer its
Fig. 10. Gasi
cation channels in coal seems[11].
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distance, the higher the oxygen concentration; otherwise, the
lower.
In the vicinity of the combustion zone, due to the very high
temperature, the oxygen is almost exhausted in the reaction with
carbon; in loosening zone, the oxygen concentration drops to a very
low point where it almost approaches zero; in dropped out zone,
owing to the comparatively low temperature, the drop of the
oxygen concentration is slow[14].
During UCG processes, the surrounding rock acting as the
furnace walls will be affected by high temperature, and its
mechanical properties willchange with the increased temperatures.
At thesame time, stressand displacement will happenamong rocks
due to the high temperature. Gasier instability would result in
steam interruption, and incomplete contact between gasication
agents and coal. Two mechanisms can play a role in a gas transport
through the porous stratum above the gas source, viz. diffusion and
permeation. The diffusion driving force is the composition gradient
(expressed through gas component mole fractions); the driving
force for permeation is the total pressure gradient.
It was found that the pressure increase inuences the speed of
the gas front movement more signicantly than the temperature
increase that is almost negligible. Nevertheless, for all tested
conditions CO2appears at the distance of the few hundred metersafter some years only. The direct proportionality of the effective
permeability coefcient to the effective squared mean pore radius
was conrmed[77].
2.2.10. Velocity of combustion front
In packed bed gasication, the combustion front moves slowly
down the bed parallel to the ow of gases. Hot combustion gases
always have intimate contact with the unburned coal ahead of the
combustion zone until the re breaks through to the production
well. In channel gasication, the combustion zone moves outward
at nearly right-angles to the ow of air and combustion gases.
During UCG a thermal wave is formed which gradually travels
through the coal bed toward the gas production well. The shape of
the thermal wave tends to change very little. Since the shape of thewave remains unchanged, the processes occurring at each
temperature level in the moving wave remain unchanged in time,
and an apparent steady-state or psuedo-steady-state condition
prevails. Under these conditions in a one-dimensional system, it is
possible to transform the mathematical model to a moving coor-
dinate system which converts partial differential to ordinary
differential equations, a major simplication of the problem. This
transformation is[78]:
n x e vt
Where
x xed spatial coordinatet time
v velocity of thermal wave or combustion front
n coordinate system moving with frontal velocity v
When the physical properties of coal tend to vary widely over
short distances even in a single coal seam making the task of
modeling such as UCG process very complex. Gasication of typical
9 m seam of sub-bituminous coal proceeds at a rate of 0.3e0.6m/
day consuming all the coal in a swath 12 to 15 m wide for a well
spacing of approximately 18 m.
2.2.11. Compositions of syngas
The precise proportions of the various component gases in any
particular syngas mixture are a function of quality and rank of
coal, seam depth, steam: oxygen ration and oxygen injection rate
and other parameter discussed in Section 2. Compositions of
syngas from a variety of coals as reported in literature reveals
component fractions in the following ranges [8,18,26,79e81].
At constant steam/oxygen ratio gas compositions remained
stable[8].
H2:11e35%; CO: 2e16%; CH4: 1e8%; CO2: 12e28%; H2S:0.03e
3.5%.
2.2.12. Optimization of UGC operation
Underground gasication cannot be controlled to the same
extent as a surface process as the coal feed cannot be processed. The
UCG process can be operated with stability and exibility, as input
ow has been shown to have a direct relationship to production
ow, with little effect on product gas quality. The power output
from the gasier could be rapidly increased or reduced by
increasing or decreasing the O2 ow rate. Although elevated depth
and pressure are not pre-requisites for a high quality gas, the
benet is in higher mass ows and hence greater efciency of
energy transmission to the surface. The energy output of a UCG
system depends on the ow rate of gaseous products and the heat
value of the gas mixture. The volume ow of the product gas is
typically four times the injection ow so the limiting factor is the
dynamic resistance of the production well.The mass ow capability
of a well is proportional to input pressure. Increasing well depth
increases the product gas density and pressure. The massow gain
due to pressure increase exceeds the frictional loss due to increased
bore hole length. Increasing the diameter of production tubing also
raises the limiting ow rate. Increasing the diameter of production
tubing, or the number of production wells, also raises the limiting
ow rate [76]. Information on the process conditions must
be constantly monitored and updated as the gasication
process moves forward. The ideal temperatures of above ground
coal gasication are about 1000 C, however, it may or may not be
possible to achieve these temperatures in UCG, primarily because
of the lack of control on water inux and reactant gas ow
patterns[57].Blinderman et al. [36,82] has used intrinsic disturbed ame
equations to determine the key parameters of the RCL process.
Wang et al. [83] performed eld trial with various operational
maneuvers, such as implementing controlled moving injection
points, O2-enriched operation and variation of operational pres-
sure to ensure the gas ow comparatively controllable and hence
improve efciency of heat and quality of the production syngas.
Lawrence Livermore National Laboratory (LLNL) is evaluating
commercial computational uid dynamics (CFD) code to model
cavity gas ow and combustion in two and three dimensions.
Fig. 11 [84] show a typical cavity conguration at a mid-to-late
stage of a linked vertical well module. Nitao et al. [84] has
provided the details of models and simulators. It will be more
useful to couple the UCG process models with full scale processsimulator so that the entire process can be modeled at once, rather
than sequentially.
3. Thermodynamics of UCG
The gasication performance is controlled by both of kinetic and
thermodynamic factors. The thermodynamic properties are, by
denition, point functions of the gasication process, indicating the
conditions of a system at equilibrium, regardless of the reaction
path followed in attaining equilibrium or the time required. On the
other hand, the kinetics of a reacting system denes a particular
sequence of reaction paths, as well as the rates at which the
chemical changes take place.
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3.1. Thermodynamic equilibrium
In gasication, both homogeneous and heterogeneous reactions
occur simultaneously in a complex reacting system [85]. As
underground coal gasication processes are thermally auto-
balanced, at given constant pressure and initial enthalpy, the
equilibrium state is reached when;
dS 0 at constantp; H (8)
Therefore, at equilibrium, when conditions of constant pressure
and enthalpy are applied, the total entropy is at maximum. Some of
the processes are at specic pressure and temperature, exothermic
or endothermic. Constraining the unit to constantTandp, we nd
that;
dG dSg (9)
and at equilibrium under these conditions, the following equation
mu